Memory devices are used to store information in semiconductor devices and systems. Resistive Random Access Memory (RRAM) cells are non-volatile memory cells that store information based on changes in electric resistance. In general, an RRAM cell includes a storage node in which a bottom electrode, a resistive switching layer and a top electrode may be sequentially stacked. The resistance of the resistive switching layer varies according to an applied voltage. An RRAM cell can be in a plurality of states in which the electric resistances are different. Each different state may represent a digital information. The state can be changed by applying a predetermined voltage or current between the electrodes. A state is maintained as long as a predetermined operation is not performed.